For measuring the filling level of a measuring medium, such as a fluid or bulk material, in a vessel, tank, container and so on, including detecting multiple medium layers, such as water and oil, and interfaces between them, it is known to use an elongated, for example a rod-shaped, probe which extends vertically over the fillable vessel height and dips into the measuring medium. The probe is longitudinally segmented into a plurality of segments, each segment comprising a sensor for measuring a predetermined physical parameter of the surrounding medium, such as its dielectric constant, impedance, temperature, magnetic or optical properties and so on. To achieve a sufficiently high measuring resolution, the number of the segments is much higher than the expected number of medium layers or interfaces. If the sensors are comprised of individual electrodes, the parameter can be measured between each sensor electrode and a common reference electrode, e.g. the conductive vessel wall, or between the respective adjacent sensor electrodes. Furthermore, if the measurement principle does not require direct contact with the medium, such as in case of a capacitance measurement in connection with a non-metallic vessel, the probe can be located outside the medium, e.g. at the outer wall of the vessel. The individual measurings, which can be performed simultaneously or successively, are finally jointly evaluated by computing means to determine the filling level and the positions of the interfaces between the different medium layers.
Level measuring devices as described above are known from U.S. Pat. No. 4,611,489, U.S. Pat. No. 6,101,873, U.S. Pat. No. 6,761,067 and WO 2007/006788.
The measurement by each individual sensor is not only dependent on the physical parameter of the immediately surrounding layer medium but also more or less affected by the physical parameters and the properties of all other layers. In addition, neither the number nor the thickness nor the physical parameters of the medium layers are known a priori. This all makes it very difficult to properly detect multiple layers and the positions of the interfaces between them and to determine the physical parameter of each layer.